In a typical three-phase electric energy meter current input signals are derived from the three phases with current transformers and voltage input signals are derived from the three phases with a resistive voltage divider. The current and voltage input signals are sampled and the current samples are multiplied with the voltage samples to obtain electric energy samples which are cumulated to provide an indication representative of consumed electric energy.
In an advanced electric energy meter the current and voltage input signals are converted to digital input samples for further processing by a microcontroller. One straight-forward approach is to use separate input channels, each for one of the three current or voltage input signals and each with an analog-to-digital converter (ADC). In this “synchronous” approach all input signals are processed in parallel and synchronously. With high accuracy requirements over a large dynamic range, e.g. smaller than 1% over a range of 1:2000, high resolution (at least 16-bit) ADCs are needed that are usually implemented with a sigma-delta modulator followed by a decimation filter. While the approach promises to be successful, it requires a large die space and is expensive. An alternative approach is to use a single high resolution ADC with an input multiplexer and an output de-multiplexer. In this “sequential” approach the current and voltage input signals are sequentially switched to the input of the ADC and the resulting digital samples are corrected in phase to compensate for the delays introduced by the sequential sampling. The sequential approach needs less die space, but requires a complex analog-to-digital converter to combine the high resolution requirements with the need to multiplex through all current and voltage signals.